docs: def tokens(self):
          """Return a list of token Spans for this sentence."""
          spans = self.word_tokenizer.span_tokenize(self.text)
          toks = [Token(
              text=self.text[span[0]:span[1]],
              start=span[0] + self.start,
              end=span[1] + self.start,
              lexicon=self.lexicon
          ) for span in spans]
          return toks